1. Field of the Invention
The present invention relates to a multi-beam image forming apparatus for forming an image based on image data transmitted from a host apparatus or the like. Particularly it relates to a multi-beam image forming apparatus suitable for wide printing of the image.
2. Description of the Background Art
An electrophotographic image forming apparatus such as a laser printer, a digital copying machine, etc. forms an image on a sheet of paper by a method including the steps of: forming an electrostatic latent image corresponding to recorded information on a photoconductor by a laser beam output from a beam generating unit after electrostatically charging a surface of the photoconductor evenly; developing the electrostatic latent image with toner to form a toner image; transferring the toner image to a sheet of paper by using a transfer portion; and fixing the toner image by using a fixing portion.
As this type image forming apparatuses, there have been heretofore proposed various multi-beam image forming apparatuses using a polygon mirror for directing laser beams toward scan lines simultaneously to form an image. The multi-beam image forming apparatus has an advantage that an image can be formed at a high speed by use of a low-speed rotating polygon motor and a low-power semiconductor laser because an image corresponding to multiple scan lines can be formed by one surface of the polygon mirror.
Use of an ASIC as an image writing means for the image forming apparatus has become the mainstream with the recent advance of semiconductor manufacturing technology. Provision of the ASIC as a general-purpose ASIC to be used in various image forming apparatuses makes mass production and drastic cost-cutting possible.
On the other hand, there has been recently a demand for a laser beam high-definition image forming apparatus, for example, using a wide sheet of paper having a sheet size of more than 20 inches. In the case of 20 inches and 1200 dpi, 24000 dots are simply required as the number of image data in the primary scanning direction. If bit depth is taken into consideration, the number of image data increases to twice or three times. It has been necessary to design a product to use the background-art ASIC for forming such a wide image in accordance with the increase in the number of image data.
As such a wide image forming method, there is a method using the background-art ASIC effectively by dividing an image forming area into parts on a photoconductor. FIGS. 6A to 6C are schematic configuration views of optical scanners applied to the background-art image forming apparatus. In FIGS. 6A to 6C, the reference numeral 1 designates a beam generating unit; 2, a polygon mirror as a scanning unit; 3, an fθ lens; and 4, a drum-shaped photoconductor. Each of the optical scanners has at least one beam generating unit 1, at least one polygon mirror 2, and at least one fθ lens 3.
Each of the optical scanners shown in FIG. 6A and 6B has two beam generating units 1, and two polygon mirrors 2. The optical scanner shown in FIG. 6C has two beam generating units 1, and one polygon mirror 2.
The optical scanner shown in FIG. 6A is configured to rotate the two polygon mirrors 2 in the same direction so that laser beams output from the beam generating units 1 are scanned in the same direction. For this reason, scan positions of laser beams can hardly be aligned accurately in the primary scanning direction because the second scan start position needs to coincide with the first scan end position. There is another technical problem that scan positions of laser beams cannot be aligned in the secondary scanning direction unless rotations of the two polygon mirrors 2 are synchronized with each other.
As shown in FIG. 6B, the optical scanner described in JP-A-6-208066 is configured to rotate the two polygon mirrors 2 in opposite directions to scan laser beams from the center toward opposite ends. For this reason, scan positions of laser beams can be aligned easily in the primary scanning direction. There is however a technical problem that scan positions of laser beams cannot be aligned in the secondary scanning direction unless rotations of the two polygon mirrors 2 are synchronized with each other.
An optical scanner described in JP-A-8-72308 is configured to rotate two polygon mirrors 2 by one drive source to synchronize rotations of the two polygon mirrors 2 with each other. It is however practically difficult that the two polygon mirrors 2 requiring high-speed rotations are rotated simultaneously by one drive source.
As shown in FIG. 6C, another optical scanner described in JP-A-8-72308 is configured so that laser beams from two beam generating units 1 are made incident on different planes of polarization of one polygon mirror 2 and joined together in the primary scanning direction on the photoconductor 4. In this configuration, scan positions of laser beams can be aligned easily in the secondary scanning direction because only one polygon mirror 2 is provided. It is however difficult to accurately align scan positions of laser beams in the primary scanning direction because laser beams are scanned in the same direction so that the second scan start position need to coincide with the first scan end position.
In the aforementioned background-art configurations, it is technically difficult to align scan positions accurately though rotations of two polygon mirrors need to be synchronized with each other. Even when only one polygon mirror is provided, there is a technical problem that it is difficult to align scan positions accurately in the primary scanning direction because laser beams from two beam generating units are scanned in the same direction.